1. Field of the Invention
The invention generally relates to CATV systems, and more specifically to an addressable system which significantly complicates the task of a pirate of receiving unauthorized service.
2. Description of the Prior Art
Numerous techniques have been proposed for preventing unauthorized users from receiving CATV programming. See, for example, U.S. Pat. Nos. 4,115,807 to Pires; 4,130,833 to Chomet; 4,268,859 to Ost; and 4,325,078 to Seaton et al.
With enough persistence and some knowledge, many of the anti-pirating techniques adopted by cable operators can be defeated. Often, once a method is devised by a pirate to receive unauthorized CATV service channels, a corresponding product is illicitly distributed to the public which allows a large number of potential customers to obtain unauthorized service. Such piracy can have serious ultimate impact on the level of service which a CATV franchise can afford to provide a community.
Modern cable television (CATV) systems employ in-home converters which can accept and process data emanating from a computer in the CATV business office. This digital data conveys commands which can enable/disable a converter, control viewability of channels and implement other ancillary functions as well. A CATV system in which a computer (addressing computer) can transmit commands to converters and the converters cannot return data to the computer is called a one-way addressable system.
Typically, digital data is conveyed on a one-way addressable cable system using a technique called Frequency Shift Keying (FSK). FSK employs two closely spaced Radio Frequency (RF) carriers to convey each of the two possible digital logic levels. For example, these two carriers can be separated by 150 KHz. The lower of the two frequencies or "SPACE" frequency is transmitted to convey digital logic level 0. Conversely, the upper or "MARK" frequency is transmitted to convey digital logic level 1. As described, the original pattern of ones and zeros output by the addressing computer is converted into an RF signal by an FSK modulator. Within the TV set-top converter in each subscriber's home an FSK demodulator is used to convert the received RF signal into the original pattern of ones and zeros produced by the addresing computer. A one-way addressable system which uses an FSK signal to convey commands is illustrated in FIG. 1.
An easy way for a subscriber to attain unauthorized service (view all channels) on a one-way addressable system which employs an FSK signalling scheme is illustrated in FIG. 2. The subscriber initially subscribes to full service for a minimal period of time. After commands are transmitted to the converter to provide full service, the customer installs an inexpensive filter to block the FSK carriers. The customer then requests the lowest-cost level of service. Since a filter has been installed, the commands changing the level of service are never received by the converter, causing it to continue to provide full service. This form of piracy can be eliminated by simply designing the converter to expect an FSK signal to be present at all times and for the signal to always convey valid bits of data. If an FSK signal is not present, or is not conveying valid bits of data, the converter becomes disabled.
Even if a converter were designed to always require the presence of a carrier frequency and the reception of valid bits of data, other opportunities for signal theft still exist.
The following three-step process could be followed by CATV subscribers to obtain unauthorized programming from a one-way addressable converter that employs FSK signals for conveying commands:
1. The subscriber connects the cable entering his home to the input of a TV signal splitter/combiner. One output of the splitter/combiner is connected to the converter. The second output of the splitter/combiner is connected to an FSK demodulator and audio tape recorder. This equipment configuration is illustrated in FIG. 3. (Alternately, the subscriber can connect the audio tape recorder directly to the `DATA` output of the FSK demodulator internal to the set-top converter.) This configuration permits the subscriber to record all CATV addressing computer commands on an audio tape.
2. The subscriber then requests full service and records on the audio tape recorder all addressing computer commands as received.
3. The subscriber configures a notch filter, his or her audio tape recorder (playback mode), FSK modulator and splitter/combiner as shown in FIG. 4. The subscriber then requests lowest-cost service.
After completion of Step 3, the converter no longer receives commands from the addressing computer (due to the notch filter) and therefore does not change the level of service. Furthermore, because of the data stored on the audio tape, the converter continues to receive commands which provide full service to the subscriber.
This piracy technique requires that the subscriber have an FSK modulator that operates at the appropriate frequencies, an audio tape recorder, a notch filter and a TV signal splitter/combiner. Note that the subscriber requires absolutely no knowledge of the bit encoding scheme or command structure of the baseband addressing signal and therefore is not required to provide intelligent circuitry to decode and process commands. This piracy scheme will work with any one-way addressable system that employs FSK modulation of two RF carriers and sends data at audio rates.